VEGETABLE CROP UPDATE

TOMATO & PEPPER EDITION

Janice LeBoeuf, OMAFRA Vegetable Crop Specialist

·         Managing late blight in organically produced tomato

For organic tomato growers, managing late blight can be a challenge.  I’m providing some information below from Margaret Tuttle McGrath, Cornell University, on managing late blight in organically produced tomato.  Some copper fungicides are registered for late blight on tomatoes, but check with your certifying body and processor before applying.  Always read and follow label instructions.

There is good information here for conventional growers as well.  Step 6 has some excellent information on removing infected plants safely.

   Managing late blight in organically produced tomato

Excerpted from Margaret Tuttle McGrath, Associate Professor, Plant Pathology, Cornell University

Late blight is a potentially very destructive disease that fortunately occurs very sporadically in most areas of the northeastern US most growing seasons.  Typically potato is the main crop affected because infested tubers are the main source of initial inoculum.  Infected tomato transplants are another potential source.  Late blight can destroy a crop if unmanaged.  The pathogen is well named: ‘Phytophthora’ in Latin means ‘plant destroyer’.   Affected foliage tissue is quickly killed.  Impact is especially great when stems are infected because all tissue above this point will die.  Additionally fruit at any stage are susceptible.  This disease can be explosive especially under favorable conditions because the pathogen can produce a lot of wind-dispersed spores and it can cycle very quickly, progressing from infection to new lesion (spot) producing spores in 6 to 7 days.  Many images of symptoms are available on the internet to assist with identification.  Mine are posted at:

www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/lateblight_tomato.htm

Steps for managing late blight in organically produced tomato:

1.   Use transplants produced in an area where late blight is not developing on plants inside or near the greenhouse. Some strains of the late blight pathogen can infect petunia and some solanaceous weeds such as bittersweet nightshade. Inspect transplants carefully before planting to ensure none have symptoms of late blight.  

2.   Control volunteer tomato and potato plants as well as solanaceous weeds, in particular hairy nightshade and bittersweet nightshade, which are also susceptible.

3.   Regularly inspect tomato as well as potato crops, which are also susceptible, for symptoms of late blight.

4.   Check local extension newsletters each week for information about late blight occurrence.  Note that during cloudy conditions spores of the late blight pathogen can survive being dispersed in wind currents long distances (miles!) because they are protected from the killing effects of UV radiation.  Rain can bring these spores down on to plants far from the affected plants that were their source.

5.   When there is a risk of late blight occurring and fungicide applications are going to be used as a component of management, apply approved fungicides on a regular preventive schedule. Late blight is difficult to control, especially when fungicides are not applied before disease onset.  Thorough spray coverage is critical with contact fungicides.  See section at end if any fungicide will be used.

6.   If symptoms of late blight are found in isolated areas in a planting, it may be possible to save the crop.  Success depends on how early in disease development symptoms are found, how many infections are present that have not yet resulted in symptoms (spore germination to symptom takes about 1 week), how quickly and thoroughly diseased tissue will be removed, environmental conditions, proximity to other gardens or farms where late blight is developing, and what management steps will be taken.  Immediately remove affected plant tissue. It is best to do this in the middle of a sunny day after the leaves have dried when there will be fewer spores and those dislodged in the process will likely be exposed to UV radiation.  Put affected tissue in garbage bags, dig a hole and bury it, or put it in a pile and cover with a tarp.  Heat that develops from sunlight hitting the tarp will quicken death of plant tissue and the pathogen.  Inspect plants daily thereafter for a week in order to find any additional affected plants that develop symptoms, then return to inspecting at least once a week. Apply approved fungicides every 7 days or as indicated on the label until final harvest or the crop is destroyed.  It is not possible to control late blight by solely relying on removing affected tissue. Even when rain is not occurring, dew over night can provide a sufficient leaf wetness period for infection.  Especially when conditions are favorable it may not be possible to control late blight with organically approved fungicides.  Monitor disease development and be prepared to jump to step 8 below.

7.   Work in affected fields last.  Clean equipment between fields.

8.   When late blight starts to become severe the foliage should be destroyed to eliminate the planting being a source of spores for other tomato or potato plantings on the farm or other farms. Propane flamers are a good way to quickly kill foliage, but are not suitable where tomatoes are grown with straw or plastic mulch or trellised.  This is an obligate pathogen that needs living host tissue to survive.  To initiate plant death, go through the planting and cut all main stems at the base, then come back through and cut stems further up in the canopy.  Disturb foliage as little as possible to minimize the amount of spores dislodged.  It is best to do this work in the middle of a sunny, preferably calm day after any moisture on leaves has dried to minimize the quantity of spores and also their likelihood of survival in the process.   Next remove trellising line and stakes, then flail chop.  

The late blight pathogen cannot survive on stakes, therefore it is not necessary to trash or even disinfect the stakes to manage this disease.  Stakes should be disinfected however, especially if bacterial diseases also developed in the planting. 

9.   Fruit from an affected field can develop symptoms after harvest and thus should be inspected just before marketing.

Additional Information About Fungicides.

Check with local organic certifying agency to determine what products are approved.  There is limited data from replicated experiments on efficacy for late blight of products approved for organic production.  Copper has provided some control where other products have failed.  However, copper is not considered very effective because it has provided poor control in efficacy experiments where excellent control was achieved with conventional fungicides.  Poor efficacy, combined with the fact that established spots, being uncontrollable with copper, will continue to produce spores, plus the explosive nature of late blight, is why a preventive spray program is recommended including by organic growers in areas where late blight occurs regularly.

Before using any fungicides read the label.

Article courtesy of Margaret Tuttle McGrath, Cornell University.

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